A fuel cell includes an electrolyte layer and a pair of electrodes placed on either side of the electrolyte layer, and generates electricity through an electrochemical reaction between fuel gas such as hydrogen and alcohol and oxidizing gas such as oxygen and air, which are supplied to the corresponding electrodes, with the aid of a catalyst. Depending on the electrolytic material used for the electrolyte layer, the fuel cell may be called as the phosphoric acid type, solid polymer type or molten carbonate type.
In particular, the solid polymer electrolyte (SPE) type fuel cell using an ion-exchange resin membrane for the electrolyte layer is considered to be highly promising because of the possibility of compact design, low operating temperature (100° C. or lower), and high efficiency.
The SPE typically includes an ion-exchange resin membrane made of perfluorocarbonsulfonic acid (Nafion: tradename), phenolsulfonic acid, polyethylenesulfonic acid, polytrifluorosulfonic acid, and so on. A porous carbon sheet impregnated with a catalyst such as platinum powder is placed on each side of the ion-exchange resin membrane to serve 5 as a gas diffusion electrode layer. This assembly is called as a membrane-electrode assembly (MEA). A fuel cell can be formed by defining a fuel gas passage on one side of the MEA and an oxidizing gas passage on the other side of the MEA by using flow distribution plates (separators).
Typically, such fuel cells are stacked, and the flow distribution plates are shared by the adjacent fuel cells in the same stack. When forming such a stack, it is necessary to seal off the passages defined on the surfaces of the MEAs from outside. Conventionally, gaskets were placed in the periphery of the interface between each adjoining pair of a MEA and a distribution plate. The contact area between the MEA and the gas diffusion electrode was ensured by pressing them together by applying an external force, typically with the aid of a suitable fastener. The required electric connection between the gas diffusion electrode and an electrode terminal connected to an external circuit was also ensured by pressing them together by applying an external force.
However, because the MEA changes its volume depending on the water content and temperature of the SPE, the external force applied by a fastener inevitably changes, and this may impair the sealing capability of the assembly. The SPE may be surrounded by a frame to stabilize the shape of the SPE, but because the frame and flow distribution plates thermally expand and contract individually, the external force applied by the fastener still changes. The change in the external force in this case produces stresses in the various members, and this may impair the durability of the various members of the assembly.
The packaging and/or the arrangement for ensuring such a controlled pressure and a required sealing performance tends to be large in size, and this has prevented a compact design for the fuel cell assembly. Furthermore, even with a highly elaborate arrangement for ensuring a sealing performance, due to the uneven thermal expansion and contraction of various parts, it has been difficult to maintain the required sealing performance for an extended period of time.